This invention relates to an integrated process for the manufacture of polymer grade 2,6-naphthalene dicarboxylic acid (2,6-NDA). Moreover, the invention allows an economic route to 2,6-naphthalene dicarboxylic acid, the preferred monomer for the production of polyethylenenaphthalate (PEN), via relatively inexpensive feedstock comprising methylnaphthalene feedstock. Methylnaphthalene is found in substantial concentration in cracked light gas oil (CLGO) produced as a by-product of ethylene production when heavy gas oil is the feedstock to the ethylene production unit. Methylnaphthalene is also produced in petroleum refineries, both in the light gas oil produced in refinery catalytic cracking operations, and in the bottom fractions of reformate from catalytic reforming operations. Coal tar, such as that produced in the steel industry, also contains methylnaphthalene. Another source of methylnaphthalene is the heavy aromatic stream generated by the UOP-BP Cyclar(copyright) process that converts liquified petroleum gases to aromatics.
The process disclosed herein is unique in many respects. Of particular importance, the new process can operate using relatively impure methylnaphthalene feedstock with respect to organic hydrocarbon impurities, allows for debromination of the oxidation product in the liquid phase, and avoids the isolation of purified naphthoic acid. It also produces the preferred monomer, naphthalene dicarboxylic acid (NDA), rather than naphthalene dicarboxylate (NDC), and optionally allows linkage to a PEN process without isolation and drying of NDA crystals.
Naphthalene dicarboxylic acids are useful as intermediates leading to various industrial chemicals, dyestuffs and the like. Polyesters prepared from 2,6 naphthalene dicarboxylic acid and ethylene glycol, such as polyethylenenaphthalate (PEN), have excellent heat resistance, gas barrier, and mechanical properties as compared with polyethylene terephthalate. Films, fibers and other shaped articles prepared from PEN display improved strength and thermal properties relative to other polyester materials. High strength fibers made from PEN can be used to make tire cords and films made from PEN are advantageously used to manufacture magnetic recording tape and components for electronic applications. In order to prepare high quality PEN it is desirable to use purified 2,6-naphthalene dicarboxylic acid.
Methods for producing 2,6 naphthalene dicarboxylic acid are known in the art. One method is the oxidation of an alkyl or acyl substituted aromatic compound to the corresponding aromatic carboxylic acid using a heavy metal catalyst in the liquid phase. For example, U.S. Pat. Nos. 2,833,816; 3,856,855; 3,870,754; 4,933,491; and 4,950,786 disclose such oxidation methods.
In U.S. Pat. No. 3,856,855 there is disclosed a process for the preparation of NDA which consists essentially of oxidizing dimethylnaphthalene with molecular oxygen at a temperature within the range of 100xc2x0 C. to 160xc2x0 C. under an oxygen partial pressure of from about 2 to 8 atm in acetic acid of an amount of at least 4 parts by wt. per part by wt of dimethylnaphthalene in the presence of a Co/Mn/Br catalyst.
More recently, U.S. Pat. No. 5,292,934 disclosed a method for preparing an aromatic carboxylic acid by oxidizing an aromatic compound having at least one oxidizable alkyl or acyl group with oxygen in the presence of a low molecular weight carboxylic acid solvent and a heavy metal at a temperature of about 250xc2x0 F. to 450xc2x0 F., heating the product to about 550xc2x0 F., and recovering the carboxylic acid.
The primary disadvantage of the methods that involve direct oxidation to 2,6NDA, is that impurities are trapped in the 2,6NDA oxidation product which forms upon oxidation as a solid in the oxidation solvent. In order to remove these impurities to a sufficiently low level acceptable for polymerization, the 2,6NDA product must be purified via multiple steps. These steps typically involve esterification, so that the resulting end product is 2,6naphthalene dicarboxylate, an ester, rather than the preferred 2,6napthalene dicarboxylic acid. Another disadvantage is that the methods referenced above require expensive base feedstocks and subsequent difficult synthesis reactions in order to prepare the feed for the oxidation to crude 2,6NDA.
Alternatively, naphthalene monocarboxylic acid and naphthalene dicarboxylic acids other than 2,6-naphthalene dicarboxylic acid can be converted to 2,6-NDA using a disproportionation reaction in the case of the monocarboxylic acids or a rearrangement reaction in the case of other naphthalene dicarboxylic acids. Henkel and Cie first patented a reaction of naphthoic acid salts to 2,6NDA in the late 1950s. (See U.S. Pat. No. 2,823,231 and U.S. Pat. No. 2,849,482).
U.S. Pat. No. 2,823,231 discloses a method of producing naphthalene 2,6-dicarboxylic acid from a naphthalene-monocarboxylic acid which comprises converting said naphthalene-monocarboxylic acid into a corresponding mono-alkali metal salt, heating said mono-salt to a temperature above about 360xc2x0 C. and below the temperature at which substantial decomposition of the starting material and reaction product takes place, in an inert atmosphere of carbon dioxide and nitrogen and converting the dialkali metal salt of naphthalene 2,6-dicarboxylic acid formed thereby into free naphthalene 2,6-dicarboxylic acid by acidification of said di-alkali metal salt with a strong mineral acid.
Other patents have claimed improvements in particular aspects of the rearrangement type of reaction. U.S. Pat. No. 4,820,868 claims improvements in yield of 2,6-naphthalene dicarboxylic acid for specific isomers subjected to a rearrangement reaction.
U.S. Pat. No. 3,766,258 claims an improvement in yield in a process for carboxylation of metallic salts of aromatic mono-, di-, or polycarboxylic acids in the presence of a metal halide catalyst and acid binding agent, the improvement consisting of using, in addition to a heavy metal catalyst, a basic metal carbonate comprising cupric carbonate or chromium carbonate.
U.S. Pat. No. 3,671,578 discloses a process for preparing 2,6-naphthalene dicarboxylic acid which enables the re-use of alkali in the alkali salt of the starting naphthalene carboxylic acid formed by the rearrangement reaction.
U.S. Pat. No. 3,952,052 discloses separation of metal salts of polycarboxylic acids by flash evaporation of the dispersant, where the dispersant is selected from biphenyl, terphenyl, quaterphenyl, and isomers and mixtures thereof.
U.S. Pat. No. 3,631,096 claims improvements in the yield of polycarboxylic acid salts by carrying out the transformation process in an inert atmosphere and in the presence of a transformation catalyst having present therewith as an adjuvant an ammonium salt of an aromatic acid.
There have been a number of references in the art which describe work relating to improved methods of purifying crude 2,6-naphthalene dicarboxylic acid. U.S. Pat. No. 3,888,921 provides a method for purifying a crude 2,6 NDA by preparing a dialkali salt, precipitating 40 to 97 mol %. of the dialkali 2,6 NDA as a monoalkali salt, and converting the precipitate to 2,6NDA.
The production of terephthalic acid provides additional relevant information regarding the art in this field. For example, U.S. Pat. No. 4,430,511 discloses an improvement in a method of producing terephthalic acid which comprises forming aggregates of crystals by direct precipitation and mixing the crystals with terphenyl to form a low viscosity slurry and transporting the slurry to a disproportionation reactor.
In the art relating to producing terephthalic acid it is also known to remove contaminants from the catalyst used in the process. For example, in U.S. Pat. No. 4,263,451, carbon impurities are removed from Zno and oxides of carbon by passing the effluent through a filter which collects the oxides of carbon.
U.S. Pat. No. 2,927,130 provides a method of recovering terephthalic acid from an aqueous solution containing alkali salts of terephthalates.
Canadian Patent 864587 discloses a process for the preparation of 2,6-NDA which comprises heating a monoalkali salt of 2,6-NDA in water or water-containing organic solvent causing disproportionation thereof into 2,6-NDA and dialkali salt and separating the 2,6-NDA.
In uk 1 472 777 it is claimed that the surface area of 2,6-NDA crystals is critical in producing pen with superior properties, including high softening point and improved color, and a method is provided for producing the specific crystals.
Currently, the most common process for making 2,6NDA starts with relatively expensive o-xylene and butadiene feedstocks, as discussed, for example, in U.S. Pat. No. 5,510,563 and U.S. Pat. No. 5,329,058 and incurs substantial yield losses of these starting materials. Following the synthesis and purification of 2,6dimethylnaphthalene (2,6 DMN), 2,6 DMN is oxidized to produce crude NDA product which forms as a solid with impurities trapped within. Therefore, in such processes, esterification to naphthalene dicarboxylate (NDC) is necessary to eliminate the impurities, as discussed in U.S. Pat. Nos. 5,254,719 and 4,886,901. Direct purification of the crude NDA via hydrogenation has been suggested by U.S. Pat. No. 5,292,934, but requires a difficult and expensive high temperature hydrogenation in the presence of a solvent. Another proposed purification scheme requires the use of nitrogen- containing species. (See U.S. Pat. No. 5,770,764 and U.S. Pat. No. 5,859,294). Crystal size and morphology is important in either case, whereas the novel process disclosed herein can optionally avoid the issue of controlling particle size of the final product.
Currently NDC is commercially available, but NDA is not, presumably because of the difficulty of producing polymerization grade NDA without esterifying to NDC. Ideally, if NDA were available commercially at a competitive price, NDA would be preferred over NDC as the starting monomer for PEN. Alternative routes to NDA based on the rearrangement reaction have been plagued with difficulties associated with handling solids, the inefficient recycling of potassium, and ineffective integration from feedstock through final product. Although various improvements have been suggested over the years, there is still a distinct need in the art for an economical, integrated process for producing 2,6-NDA, the preferred monomer for the production of polyethylenenaphthalate (PEN).
In accordance with the foregoing, the present invention is an integrated process for producing 2,6-naphthalene dicarboxylic acid, which comprises:
a) Reacting a hydrocarbon stream containing predominantly methylnaphthalene with an oxygen-containing gas in the presence of a suitable solvent and catalyst to form a crude mixture of naphthoic acid (crude product NA), wherein said crude product NA remains dissolved in the solvent;
b) Recovering said crude product NA by evaporation of solvent and washing said crude product NA with water;
c) Debrominating said crude product NA by passing over a supported catalyst in the presence of hydrogen, and water-washing said crude debrominated product NA;
d) Contacting said crude, debrominated product NA with an aqueous base of potassium to extract pure NA as the aqueous potassium salt of NA;
e) Separating said aqueous potassium salt of NA from the remaining organic liquid (containing methylnaphthalene and partially oxidized reaction intermediates), and recycling said organic liquid to step a);
f) Contacting said aqueous potassium salt of NA with naphthalene vapor, adding a solid catalyst, and removing water by evaporation to form a slurry of solid potassium salt of NA and catalyst suspended in liquid naphthalene;
g) Reacting said slurry in the presence of carbon dioxide to convert solid potassium salt of NA to liquid naphthalene and solid dipotassium salt of 2,6-NDA(2,6-K2NDA);
h) Reducing the pressure to vaporize the naphthalene, and separating the solids from the naphthalene vapor by a novel separation using cyclones, recycling a portion of the naphthalene to step (f), and recovering the remainder as a product, or methylating the naphthalene via direct alkylation or transalkylation to provide additional methylnaphthalene feed for step (a);
i) Contacting the solids with water to create a mixture of aqueous potassium salts (comprising the potassium salt of NA, KNA, and the dipotassium salt of 2,6-NDA, 2,6-K2NDA, and its isomers) and solid catalyst;
j) Separating the solid catalyst from the mixture of aqueous potassium salts and recycling it to step (f);
k) Adding aqueous potassium bicarbonate to the mixture of aqueous potassium salts and evaporating a portion of the water to selectively crystallize the dipotassium salt of 2,6-NDA as a solid, separating said solid, and recycling the remaining liquid to step (d);
l) Dissolving the solid dipotassium salt of 2,6-NDA in water;
m) Optionally passing said aqueous dipotassium salt of 2,6-NDA through an activated carbon bed to remove impurities;
n) Contacting said aqueous dipotassium salt of 2,6-NDA with carbon dioxide to create a mixture of solid monopotassium salt of 2,6-NDA and aqueous potassium bicarbonate, separating said solids, and recycling the aqueous potassium bicarbonate to step (k);
o) Contacting solid monopotassium salt of 2,6-NDA with water, optionally in the presence of carbon dioxide, to form solid 2,6-NDA, aqueous dipotassium salt of 2,6-NDA, and potassium bicarbonate;
p) Separating the solid 2,6-NDA and recycling the liquid containing aqueous dipotassium salt of 2,6-NDA and potassium bicarbonate to step (n);
q) Contacting solid 2,6-NDA with water in a pipe reactor to remove traces of potassium ion impurity;
r) Separating solid 2,6-NDA and recycling water to step o);
s) Washing the solid 2,6NDA with additional water
t) Separating the water from the solid, producing wet polymer grade 2,6NDA, and recycling most of the water to step q)
u) Drying solid 2,6-NDA by conventional means, or optionally feeding as an aqueous slurry to a process for making PEN, optionally adding additional water if necessary.
The integrated process produces the preferred 2,6-NDA isomer, and demonstrates many novel steps and advantages over any process available in the art.